This invention relates to the art of thermal heads for thermal recording which are used in various types of printers, plotters, facsimile, recorders and the like as recording means.
Thermal materials comprising a thermal recording layer on a substrate of a film or the like are commonly used to record images produced in diagnosis by ultrasonic scanning (sonography).
This recording method, also referred to as thermal recording, eliminates the need for wet processing and offers several advantages including convenience in handling. Hence in recent years, the use of the thermal recording system is not limited to small-scale applications such as diagnosis by ultrasonic scanning and an extension to those areas of medical diagnoses such as CT, MRI and X-ray photography where large and high-quality images are required is under review.
As is well known, thermal recording involves the use of a thermal head having a glaze, in which heating elements comprising heaters and electrodes, used for heating the thermal recording layer of a thermal material to record an image are arranged in one direction (main scanning direction) and, with the glaze urged at small pressure against the thermal material (thermal recording layer), the two members are moved relative to each other in the auxiliary scanning direction perpendicular to the main scanning direction, and the heaters of the respective pixels in the glaze are heated by energy application in accordance with image data to be recorded which were supplied from an image data supply source such as MRI or CT in order to heat the thermal recording layer of the thermal material, thereby accomplishing image reproduction.
A protective film is formed on the surface of the glaze of the thermal head in order to protect the heaters for heating a thermal material, the associated electrodes and the like. Therefore, it is this protective film that contacts the thermal material during thermal recording and the heaters heat the thermal material through this protective film so as to perform thermal recording.
The protective film is usually made of wear-resistant ceramics; however, during thermal recording, the surface of the protective film is heated and kept in sliding contact with the thermal material, so it will gradually wear and deteriorate upon repeated recording.
If the wear of the protective film progresses, density unevenness will occur on the thermal image or a desired protective strength can not be maintained and, hence, the ability of the film to protect the heaters is impaired to such an extent that the intended image recording is no longer possible (the head has lost its function).
Particularly in the applications such as the aforementioned medical use which require multiple gradation images of high quality, the trend is toward ensuring the desired high image quality by adopting thermal films with highly rigid substrates such as polyester films and also increasing the setting values of recording temperature (energy applied) and of the pressure at which the thermal head is urged against the thermal material. Under these circumstances, as compared with the conventional thermal recording, a greater force and more heat are exerted on the protective film of the thermal head, making wear and corrosion (or wear due to corrosion) more likely to progress.
With a view to preventing the wear of the protective film on the thermal head and improving its durability, a number of techniques to improve the performance of the protective film have been considered. Among others, a carbon-based protective film (hereinafter referred to as a carbon protective layer) is known as a protective film excellent in resistance to wear and corrosion.
Thus, Examined Published Japanese Patent Applications (KOKOKU) No. 61-53955 and No. 4-62866 (the latter being the divisional application of the former) disclose a thermal head excellent in wear resistance and response which is obtained by forming a very thin carbon protective layer having a Vickers hardness of 4500 kg/mm.sup.2 or more as the protective film of the thermal head and a method of manufacturing the thermal head, respectively. The carbon protective layer has properties quite similar to those of diamond including a very high hardness and chemical stability, hence the carbon protective layer presents sufficiently excellent properties to prevent wear and corrosion which may be caused by the sliding contact with thermal materials.
The carbon protective layer is excellent in wear resistance, but brittle because of its hardness, that is, low in tenacity. Heat shock and a thermal stress due to heating of heating elements may bring about rather easily cracks or peeling-off.
In order to resolve the problem, Unexamined Published Japanese Patent Application (KOKAI) No. 7-132628 discloses a thermal head which has a dual protective film comprising a lower silicon-based compound layer and an overlying diamond-like carbon layer, whereby the potential wear and breakage of the protective film due to heat shock are significantly reduced to ensure that high-quality images can be recorded over an extended period of time. In this application, the adhesion of the silicon-based compound layer to the diamond-like carbon layer is improved by subjecting the surface of the silicon-based compound layer to a surface treatment such as plasma-assisted CVD in a reducing atmosphere.
However, the adhesion between the two layers is not enough to protect the protective film from cracks or peeling-off which may be caused by a stress due to a difference in coefficient of thermal expansion between the respective layers, a mechanical impact due to a foreign matter entered between the thermal material and the thermal head (glaze) during recording or other factors.
As shown in FIG. 5, the top of a glaze 182 of a thermal head 166 (which is shown to face down in FIG. 5, since the thermal head 166 is pressed downward against a thermal material A) is overlaid with a heater 184 which, in turn, is overlaid with electrodes 186 provided on both sides at a specified distance, whereupon a depression step having a depth corresponding to the thickness of the electrodes 186 (usually about 1 .mu.m) is formed between the electrodes 186 and the heater 184. Therefore, after forming a protective film comprising two protective layers 188 and 190, the depression step remains as the surface geometry of the protective film. Especially, the mechanical impact due to a foreign matter entered between the thermal material and the thermal head (glaze) during recording or the like readily concentrates in the thus formed depression step on the surface of the protective film. Hence, the diamond-like carbon layer 190 will have cracks or peeling-off in the neighborhood of the depression step.
The cracks or peeling-off in the protective layer give rise to wear, corrosion and wear due to corrosion, which results in reduction of the durability of the thermal head. The thermal head is not capable of exhibiting high reliability over an extended period of time.